Electronic circuits can generally be categorized as discrete circuits, integrated circuits (IC), or monolithic microwave integrated circuits (MMIC). IC technology includes a chip which is formed from a single doped crystal. Silicon is the typical substrate used in high frequency IC applications and is sometimes used in the higher frequency microwave frequencies. Gallium Arsenide substrates are generally better suited to microwave frequencies and are thus often used for fabricating MMICs.
The recent advances made in MMIC (as well as other integrated circuit) technology are well known, and have been applied to such demanding and varied fields as communications and avionics. One of the basic components of MMIC and IC technology is the oscillating transistor, which may be defined as a transistor device that is biased to convert a DC input into an AC output, and which is especially useful in frequency generation applications, etc.
One difficulty with oscillating transistors, however, is that local temperature variations modify the frequency of the signal being generated. It is typical to maintain the oscillating transistor within a narrow temperature range if the desired frequency is to be maintained. Temperature variations may be caused by heat developed within the electronic circuit itself, or by changes in ambient temperature such as may occur from a change in the altitude of an aircraft. Accurate frequency stabilization based upon maintaining a constant local temperature of the oscillating transistor is a preferable technique for many IC and MMIC applications. Although maintaining the temperature of the oscillating transistor is preferred in many of the disclosed embodiments of the present invention, this is not required in all cases.
There are several techniques which are known to compensate for a variable temperature of an oscillating transistor. Some techniques change the temperature at which the oscillating transistor is operating at to maintain a constant output frequency. The input of the oscillating transistor can be modified to compensate for a variation in temperature. Modifying the output of the oscillating transistor typically requires a complex feedback system which must function under substantially real time conditions.
Certain prior art oscillating transistors involve applying heating elements to the transistors to establish a predetermined narrow range of operating temperatures. However, these oscillating transistors either have no temperature sensor, and/or the heating element is not interspersed with the oscillating transistor.
A heat controlled oscillating FET is disclosed in Japanese Patent 04-185106 to Fumiaki which was published on Jul. 2, 1992. In the Japanese patent, the heating elements are not interspersed with the oscillating transistor. When the oscillating FET of Fumiaki is configured to provide significant power, the components of the oscillating transistor typically becomes much larger. When the dimensions of the oscillating FET are enlarged, it becomes difficult to evenly heat the oscillating FET so that the operation of the FET can be reliably predicted. Even heating elements located on the same chip as the oscillating transistor may not control the temperature of an oscillating FET as closely as desired if the size of the oscillating FET becomes very large. Consequently, the design of the prior art oscillating transistors with heaters is often insufficient to maintain the oscillating FETs within the desired temperature range. It is also difficult to precisely sense the temperature of a relatively large oscillating transistor. The reliability and predictability of such circuits typically decrease with increasing size. Additionally, it is difficult to place the heating unit in close proximity to the oscillating FET due to the associated wiring (busses) of the components on the chip itself.
From the above, it should be evident that a device which reliably maintains the local temperature of an oscillating transistor within a narrow temperature range is desired and represents a significant improvement over the prior art.